How Life Took to Land

The fossil of a fish with legs reflects the prehistoric moment when sea life made way for land animals.

When Harvard University zoologist Farish Jenkins and a team of scientists went into the Canadian arctic, equipped with a month's supply of provisions and sabot-slug rifles to deal with nosy polar bears, he was looking for fossils of ancient land animals known as tetrapods. What he found instead was something far more significant: the fossil of a fish with legs, legs it might have used to crawl up out of the water some 375 million years ago. "You're talking to a modified fish," says Jenkins, modestly.

Tiktaalik roseae, as the land fish is called, can reach about nine feet long and, despite a distinctly fishy appearance, is equipped with rudimentary tools to let it survive on land. Its finlike appendages are sturdy enough to support its weight, with internal bone structures recognizable as the prototypes of modern mammal limbs. Its head, which resembles a crocodile's, is a separate structure from the shoulders, allowing it to crane its neck—a useful capability for feeding on land. And Tiktaalik's lungs, which evolved from the air bladders fish use for respiration and buoyancy control, are housed in a sturdy overlapping rib cage, the better to buttress the animal's weight against gravity. "No other fish in the world either living or dead has overlapping ribs," Jenkins says.

But why abandon the water in the first place? One theory is that the fish were trying to avoid predators in shallow waters, fleeing for their lives in a low-speed chase that played out over generations. "If you're a fish that can operate in very shallow water, you can escape predators that can only swim in deeper water," Jenkins explains. Or they might have been seeking new food sources among the invertebrates running around on land—and after millions of years underwater eating fish and seaweed, could anyone blame them?